The present invention relates to a fuel cell, especially a polymer electrolyte fuel cell, which is used for portable power sources, electric vehicle power sources, and domestic cogeneration systems.
The fuel cells, especially the polymer electrolyte fuel cells, cause a fuel gas such as hydrogen, and an oxidant gas such as the air, to be subjected to electrochemical reactions at gas diffusion electrodes, thereby generating the electric power and the heat simultaneously.
The structure of a conventional polymer electrolyte fuel cell is described below.
FIG. 2 is a sectional view schematically illustrating a membrane electrode assembly (hereinafter referred to as MEA) in the conventional polymer electrolyte fuel cell. A pair of catalytic reaction layers 12, which are mainly composed of carbon powder with a platinum catalyst, are closely attached to both faces of a polymer electrolyte film 11. A pair of diffusion layers 13 having both the gas permeability and the electrical conductivity are further arranged on the respective outer faces of the catalytic reaction layers 12. The polymer electrolyte film 11, the pair of catalytic reaction layers 12, and the pair of diffusion layers 13 constitute an MEA 14.
A pair of conductive separator 15 is placed at both face of the MEA 14, so that a plurality of MEAs 14 are electrically connected with one another in series. A gas flow path 15a is formed between the separator 15 and the MEA 14 in order to supply fuel such as hydrogen gas and oxidant gas to the electrode, and in order to flow out a gas generated by the electrochemical reaction and non-reacted remaining fuel gas. The gas flow path may be provided independently of the separator, but in general, grooves formed on the surface of the separator 15 function as the gas flow path 15a. A conventional example of the separator 15 is a cut piece of a plate, which is obtained by sintering glassy carbon under high pressure at high temperature.
A cooling flow path is provided on the other surface of the separator 15 to circulate cooling water and keep the temperature of the fuel cell. Circulation of cooling water enables the thermal energy generated by the reaction to be utilized, for example, in the form of hot water.
Gas sealings and O-rings are placed around the MEA 14 across the polymer electrolyte film 11, in order to prevent gasses from leaking or from mixing with each other and in order to prevent the cooling water from leaking. Gaskets that are composed of a resin or a metal plate and have substantially the same thickness as that of the MEA may also be arranged around the MEA, and the clearance between the gasket and the separator may be sealed with a grease or an adhesive.
In most cases, a large number of unit cells are laid to construct a stack structure of fuel cells. A cooling plate is provided for every one or two unit cells, in order to cool the fuel cell. The cooling plate is generally a thin metal plate in which a cooling water is flowing. Another possible configuration makes the separator itself function as a cooling plate. In this case, a water path is formed on the rear face of the separator, which includes in each unit cell. In this structure, O-rings and gaskets are also required to seal a cooling water. The O-rings in the seal should be smashed or flattened completely to ensure the sufficient electrical conductivity between the cooling plate.
In the stack of fuel cells, the conventional arrangement has an internal manifold, in which supply inlets and exhaust outlets of gases and cooling water to and from the respective unit cells are disposed inside the cell. In the case where a reformed gas is used as the fuel gas, however, the CO concentration rises in the downstream area of the flow path of the fuel gas in each unit cell. This may cause the electrode to be poisoned with CO, which results in lowering the temperature and thereby further accelerating the poisoning of the electrode. In order to relieve the deterioration of the cell performance, an external manifold is noted as a preferable configuration that increases the length of the gas supply and exhaust system between the manifold and each unit cell.
In either of the internal or the external manifold, the gas flow paths should be formed by a cutting process when a dense carbon plate or glassy carbon plate having the gas tight property is used for the material of the separators. The cutting process, however, undesirably prevents mass production of the fuel cells with a low manufacturing cost.
The carbon plate typically has porosity and thereby relatively poor gas tight property. A carbon plate impregnated with a resin is thus generally used for the separators of the fuel cells. The cured resin, however, hardly has elasticity, so that the carbon plate impregnated with the resin after the cutting process of the gas flow paths may have a warpage. It is thus required to carry out the formation of the gas flow paths after the carbon plate is impregnated with the resin. When a phenol resin or a silicone resin is used as the impregnating agent, the separator has insufficient acid resistance.
Another possible application mixes carbon powder or metal powder with a resin and manufactures a separator by compression molding or injection molding. In this case, the resin should have sufficient acid resistance. When a hard material like polytetrafluoroethylene is used for the separator, the molded separator does not have sufficient fluidity.
When the resin used as the impregnating agent has poor fluidity, it is required to decrease the content of the resin. A specific part of the molded separator that requires the gas tight property should thus be impregnated again with a resin.
The object of the present invention is to provide a compact fuel cell stack that is manufactured by a simple process.
At least part of the above and the other related objects is realized by a fuel cell stack including a plurality of unit cells. Each of the unit cells includes a polymer electrolyte film, a pair of electrodes that are arranged across the polymer electrolyte film and respectively have a catalytic reaction layer, and a pair of separators, one having a means for supplying fuel gas to one of the electrodes and the other having a means for supplying oxidant gas to the other of the electrodes. The separator has a laminate structure comprising a gas-tight conductive plate A and another conductive plate B having at least one slit, which continuously meanders from one end to another end of the conductive plate B.
In accordance with one preferable application of the present invention, the laminate structure includes at least one gas-tight conductive plate A and at least two conductive plates B, and the gas-tight conductive plate A is disposed on both outer-most layers of the fuel cell stack.
In one preferable embodiment of the present invention, the slit has an end that is not open to outside on a plane of the conductive plate B, and the fuel cell stack has an internal manifold that causes a gasses to be fed to and discharged from each of the unit cells.
In another preferable embodiment of the present invention, the slit has an end that is open to outside on a plane of the conductive plate B, and the fuel cell stack has an external manifold that is arranged on a side face of the fuel cell stack, which causes gasses to be fed to and discharged from each of the unit cells.
In still another preferable embodiment of the present invention, the conductive plate B has a lug formed by an end of the slit, and the fuel cell stack has an external manifold that is arranged on a side face of the fuel cell stack, which causes gasses to be fed to and discharged from each of the unit cells.
In this structure, it is preferable that the lug is located inside the external manifold.
It is preferable that the electrolyte is a proton-conductive polymer electrolyte.
It is also preferable that the separator has a side face sealed with a gas-tight material in the fuel cell stack.
It is further preferable that the separator has a laminating surface sealed with a gas-tight material in the fuel cell stack.
It is also preferable that the conductive plate B is a punched metal plate.